MAINTAINERS: update SiFive HiFive Unmatched maintainers

Email account pragnesh.patel@sifive.com does not exist anymore.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

board/sifive/unmatched/MAINTAINERS

 SiFive HiFive Unmatched FU740 BOARD
 M:	Paul Walmsley <paul.walmsley@sifive.com>
-M:	Pragnesh Patel <pragnesh.patel@sifive.com>
 M:	Green Wan <green.wan@sifive.com>
 S:	Maintained
 F:	board/sifive/unmatched/